setcar
As you might guess from their names, the setcar and setcdr
functions set the car or the cdr of a list to a new value.
They actually change the original list, unlike car and cdr
which leave the original list as it was. One way to find out how this
works is to experiment. We will start with the setcar function.
First, we can make a list and then set the value of a variable to the
list, using the setq function. Here is a list of animals:
(setq animals '(giraffe antelope tiger lion))
If you are reading this in Info inside of GNU Emacs, you can evaluate this expression in the usual fashion, by positioning the cursor after the expression and typing C-x C-e. (I'm doing this right here as I write this. This is one of the advantages of having the interpreter built into the computing environment.)
When we evaluate the variable animals, we see that it is bound to
the list (giraffe antelope tiger lion):
animals
=> (giraffe antelope tiger lion)
Put another way, the variable animals points to the list
(giraffe antelope tiger lion).
Next, evaluate the function setcar while passing it two
arguments, the variable animals and the quoted symbol
hippopotamus; this is done by writing the three element list
(setcar animals 'hippopotamus) and then evaluating it in the
usual fashion:
(setcar animals 'hippopotamus)
After evaluating this expression, evaluate the variable animals
again. You will see that the list of animals has changed:
animals
=> (hippopotamus antelope tiger lion)
The first element on the list, giraffe is replaced by
hippopotamus.
So we can see that setcar did not add a new element to the list
as cons would have; it replaced giraffe with
hippopotamus; it changed the list.
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